Present techniques involve the insertion of a parallel-wire or two-wire transmission line in the porous material. Here we describe improved insertion probes and new surface, non-insertion, TDR probes for volumetric water content, .theta., and electrical conductivity, .sigma., measurement. The probes can be used for .theta. and .sigma. measurement in porous solids, powders, slurries and liquids. The surface probes may be used on conveyor belts and for following drying of solids and films. The range of materials for which these probes may be used includes, but is not limited to soil, rock, coal, concrete, paper, wood, powders, oils, petroleum fluids, paints and paints and industrial slurries. It will be further understood that although the probes will be described in relation to specific measurement applications, the probes according to the invention are not limited to use in such applications.
The two-wire transmission line TDR probes currently in field use suffer from unwanted noise and information loss due to impedance mismatch between the probe and the coaxial connecting cable. Here we describe symmetric, multi-wire probes designed to minimize these problems and eliminate the need for a balancing transformer between probe and TDR device.
Analysis of the electric field distributions around these new probes shows that they emulate a coaxial transmission line and their measured characteristic probe impedances approach that of coaxial probes. Signals from the new probes permit more reliable and accurate .theta. and .sigma. measurement and are superior to those of two-wire probes with balancing transformer. The enhanced signal clarity of the new probes extends to sample diameters of at least 0.2 m. We show that electrical conductivity determined with the new probes is identical to that found with a coaxial cell and substantially different from that measured by a two-wire probe. We find that the Giese-Tiemann thin sample approach for estimating .sigma. can be adapted to these multi-wire probes and is superior to an existing measurement scheme. Non-insertion surface probes capable of determining .theta. and .sigma. close to surfaces and in thin films are also described.
The need for rapid, reliable and routine techniques for monitoring in situ volumetric water content .theta. and soil electrical conductivity .sigma. in hydrology, agriculture and in various aspects of civil engineering and industrial processes is obvious and needs no elaboration. Time Domain Reflectometry (TDR) appears to have the potential to provide these three desired characteristics, particularly for .theta.-determinations and perhaps for .sigma. as well.
The coaxial transmission line cell used in laboratory studies is unsuitable for most field soils. Instead, parallel-wire or two-wire transmission line probes are inserted into the soil. Because the geometry of the two-wire probe differs from that of the coaxial cable connecting the probe to the TDR device, considerable signal and information loss occurs at the coaxial cable/probe interface. In an attempt to match the electrical characteristics a balancing transformer, or balun, is usually connected at the interface. However this transformer can itself be a source of unwanted noise and can cause difficulties in analysing the TDR signal, particularly in saline materials. This problem becomes acute in short probes (.ltoreq.0.25 m). Since TDR is superior to other in situ techniques in the near soil-surface region, it is especially desirable to minimize uncertainty in signal analysis for shorter probes.
We describe here simple single and multi-wire probes which emulate coaxial transmission lines, eliminate the necessity for balancing transformers and reduce spurious noise and reflections. We present an analysis of the electric field around the probes and demonstrate their advantages for both water content and electrical conductivity measurement.